1. Field of the Invention
The present invention relates to polycarbonate for disc substrates, a process for producing said polycarbonate, and a disc substrate made of said polycarbonate. More particularly, it is concerned with high purity polycarbonate, of which the impurity content is greatly decreased and which is useful as a material for production of disc substrates, a process for efficiently producing said high purity polycarbonate, and a disc substrate made of said high purity polycarbonate.
2. Description of Related Art
Polycarbonate is generally used as a material for production of disc substrates such as for optical discs, magnetic discs and so forth.
The conventional polycarbonate, however, when molded and used as a disc substrate, suffers from various problems. For example, (1) adhesion force between the substrate and a recording film is insufficiently low, (2) metals such as iron, gallium and terbium, existing in the recording film are gradually corroded, and (3) a mold is corroded during molding and thus the service life of a stamper is shortened.
As a result of extensive investigations to overcome the above problems encountered in using the conventional polycarbonate as a disc substrate and to produce polycarbonate which exhibits superior performance as a disc substrate and causes no corrosion of a mold to be used in molding, it has been found that the conventional polycarbonate, particularly polycarbonate produced by the phosgene method contains, as impurities, unreacted starting materials, a solvent used, and low molecular weight polymers, and that these impurities are responsible for the above problems.
Based on the above findings, further investigations were made, and it has been found that high purity polycarbonate having a greatly decreased impurity content is obtained by treating polycarbonate containing the above impurities with a ketone such as acetone and methyl ethyl ketone (MEK), and that such high purity polycarbonate is free from the above problems and exhibits excellent performance as a disc substrate.
Furthermore it has been found that if extraction of impurities with an organic solvent such as a ketone, separation of the organic solvent and heat drying of the resulting polycarbonate are separately carried out in each apparatus, fine dust may be entrained in the polycarbonate mixture during transportation thereof from an apparatus to the following apparatus, and that if all the operations are carried out in the single apparatus, polycarbonate having much higher quality results.